This invention generally relates to metal deforming, and more particularly to tube end expanders of the roller and mandrel type.
Tube expanders of the roller and mandrel type are widely used for expanding tube ends into pressure contact with tube sheets. In their usual construction, the roller and mandrel type tube expanders include a tubular cage, a plurality of rollers lying within the cage, and a rotatable, forwardly tapered mandrel axially extending through the cage. The surface of the cage defines a plurality of slots, outer surfaces of the rollers extend through these slots, and inner surfaces of the rollers are in frictional, driving contact with the mandrel. Moreover, the forward end of each roller tapers inward toward the centerline thereof, forming a truncated cone section. With this usual construction, forward movement of the mandrel forces the rollers radially outward while rotation of the mandrel rotates the rollers and the cage. To expand a tube end, the mandrel is axially located relative to the cage and rollers so that the rollers are positioned with outer surfaces thereof defining the circle to which the interior of the tube is to be expanded. The mandrel is then rotated, rotating the rollers and cage. The cage and rollers are axially moved into the inside of the tube until the forward, tapered ends of the rollers come into contact with the inside surface of the tube. Thereafter, with some expanders, rotation of the rollers against and around the interior surface of the tube produces a threading action, pulling the rollers and the cage further into the tube, and with other expanders, the cage and rollers are forced further into the tube. In either case, the forward, tapered ends of the rollers axially move further into the tube, forcing the tube surface radially outward. When tube expansion is completed, the tube expander is withdrawn from the tube either by pulling the expander therefrom or by reversing mandrel rotation to produce a threading action pushing the rollers and cage out of the tube.
Prior art tube end expanders and methods of operating the same of the general type described above have not been completely satisfactory. For example, just as the forward, tapered ends of the rollers exert radially outward forces on the inside surface of the tube, the tube surface exerts radially inward forces on the forward ends of the rollers. These radially inward forces push the forward ends of the rollers toward the mandrel, thus tending to pivot each roller about the inside, base edge of the truncated cone section thereof. Such pivoting action raises the portions of the rollers rearward of the truncated cone sections, decreasing the area of contact, and thus the frictional forces, between the rollers and the mandrel. This contact area may become so diminished that the frictional forces between the rollers and mandrel are insufficient to rotate the rollers.
If this happens, the rollers and cage stop rotating, detrimentally affecting tube expansion. For example, because the rollers and cage stop rotating around the inside surface of the tube, tube expansion becomes non-uniform, resulting in high and low spots. These low spots may result in gaps between the exterior of the tube and a tube sheet through which the tube extends, preventing the development of a good seal therebetween. Alternately, localized sections of the tube may be excessively expanded, cracking the tube.